Subpopulations of lymphocytes may be defined on the basis of functional, morphological and immunochemical differences. Included among the properties that distinguish thymus-derived (T) vs. bone marrow-derived (B) lymphocytes are the apparent present of absence of receptor sites for specific substances (Fc of immunoglobulin, C'3, etc). and the responce of the involved cell type to interaction with mitogen (e.g., PHA, Con A, LPS). These differences between T and B cells reflect discrepancies in the chemical composition and structural arrangement of the plasma membrane. B cells may be further divided into subpopulations on the basis of surface membrane immunoglobulins. More recently, subpopulations of T cells have been defined on the basis of function and cell surface alloantigens; helper (Ly 1) and suppressor (Ly 2/3) T cells are of particular importance in this regard. Investigations to date suggest that most, if not all, the above subpopulations differ in their sensitivity to ionizing radiation. Thus: 1) irradiated B cells placed in tissue culture survive less well than T cells; 2) the capacity of irradiated cells to traffic in normal fashion is inhibited at lower dose levels for B cells than for T cells; 3) after whole body exposure, proportionately greater numbers of T than B cells continue to recirculate and maintain viability in secondary lymphoid tissues; 4) B cells demonstrate radiation-induced morphologic alterations at lower dose levels than do T cells as documented by transmission and scanning electron microscopy; 5) T and B cells activated to appropriate mitogens or antigens in vivo or in vitro are more resistant than their nonactivated counterparts to radiation-induced interphase cell death; 6) suppressor T cells appear to be more radiosensitive than helper T cells; 8) a subpopulation of cytotoxic lymphocytes (T cells) is very resistant to radiation injury. The purpose of this project is twofold: expand the above observations on the radiosensitivity of defined populations of lymphocytes; define the physical-chemical basis of these differences in radiosensitivity and thereby gain a better understanding of the pathogenesis of radiation-induced interphase death of lymphocytes.